1. Field of the Invention
This invention relates to the field of optical coatings, and particularly to optical coatings where wide field-of-view is required.
2. Description of the Related Art
Optical thin film interference coatings are used in many applications where the spectral content of light is important. When applied to a surface in an optical system, such coatings allow some wavelengths to be transmitted and other wavelengths to be reflected. Common applications include antireflection coatings that transmit all relevant wavelengths with high efficiency, bandpass filters that allow only a specified spectral region to be transmitted and to reflect other wavelengths, and notch filters where specific wavelengths are reflected while other wavelengths are transmitted. Such coatings are well known, and are described, for example, in U.S. Pat. No. 4,756,602 to Southwell et al. and U.S. Pat. No. 4,666,250 to Southwell.
All of these optical coatings suffer from an angle-of-incidence (AOI) dependent spectral response. This change in performance is a result of the changing path length, for light propagating in the optical coating, of the light as a function of the incident angle. This applies to all optical interference coatings no matter the application, including antireflection, bandpass, bandstop, and high reflection applications.
In many applications, this shift in the spectral properties of the optical coating is undesirable. In these applications, the desire is to have a very wide field-of-view where the spectral properties do not change as a function of incidence angle on the coating.
Applications where wide field-of-view are important include:
1. Optical sensors on a gimbal placed behind a window. The sensor can change its viewing direction over a wide range of angles, in each case viewing through a different area of the window. A uniform coating on the window will have different spectral characteristics for the light that impinges on the sensor depending on the direction in which the sensor is pointed. Also, light coming through any fixed portion of the window that originates from different locations will impinge on the window from different directions and have different spectral characteristics imposed on it.
2. A head mounted display (HMD) with a see-through beam combiner where the projected image is reflected into the users eyes using a fixed (e.g., CRT, LCD) or scanning (e.g., laser, laser diode) image source. These beam combiners are designed to operate at a non-normal angle of incidence and, based on their proximity to the user's eyes, can be required to provide constant spectral performance over wide viewing angles. The coatings on the beam combiner are designed to reflect specific wavelengths of the HMD display and transmit the external scene to the user. The reflected display wavelengths will have a different spectral response depending on the specific location on the beam combiner where the light is reflected. This can cause the beam combiner reflection band, as perceived by the eye, to shift relative to the display wavelengths, reducing the display intensity perceived by the user and changing the spectral content of the external scene as viewed by the wearer. These changes in intensity and spectral content will vary with location on the beam combiner as determined by the relative angle between any given location and the pupil of the eye.
3. Spectacles or sunglasses having coatings that tailor the spectral content of the transmitted light. An example might be optical coatings that are applied to sunglasses to preferentially transmit those wavelengths for which the eye has greater visual acuity. For example, these glasses are important for hunters and sportsmen. Light transmitted at different locations and at different angles would have a different spectral content, which can be perceptible to the user.
Many other applications will be readily apparent to those skilled in the art.